The anticipation of synthetic fuel and biodiesel entering the fuel supply has led to concern regarding the relative stability of Fischer-Tropsch (FT) fuel, biofuel, and their blends with petroleum. This report summarizes the storage stability results found for a stable petroleum fuel and its blends with biodiesel and FT diesel. The findings show that at even low concentration, 5%, biodiesel appears insoluble in FT and FT petroleum middle distillate blends. The insolubility produced a large yield of sediments in the ASTM storage stability test. This insolubility means the fuel could not be stored or used as diesel fuel for combat operations.